1. Field of the Invention
The present invention relates to an ultrasonic diagnosis apparatus capable of not only effectively displaying dynamic states of flow of blood in a subject to be examined, particularly, pulsation of the flow of blood, but also three-dimensionally displaying pulsatile flows of blood in the subject.
2. Description of Related Art
An ultrasonic diagnosis apparatus has normally various types of displaying images, as has been widely known, which can be used for diagnosis on ultrasound images. Such types of displaying images include a CFM (Color Flow Mapping) mode used for displaying blood flow images, as well as tomographic image modes, such as a B-mode, used for displaying tomographic images.
Among these modes, the CFM mode is prepared for displaying two-dimensional blood flow information in real time. In this mode, generally, flows of blood approaching to the ultrasound probe are displayed in red on a monitor, while flows of blood going away from the probe are displayed in blue on the monitor, so that information about blood flow is visually distinguishable.
The following describes the principle and an outline of process for displaying information in relation to blood flow on the CFM mode. As conventionally well known, an ultrasonic diagnosis apparatus obtains echo signals sequentially in time by performing ultrasound scanning at each location (direction) in a subject to be examined a plurality of times (N-times). Then, from the echo signals obtained sequentially in time, the apparatus detects information in relation to velocity and/or scattering power of blood flow at a desired depth in the scanned region on the basis of the Doppler technique. That is, scanning the same location in the subject at predetermined intervals provides Doppler signals expressed as a quantity of phase shift per unit time of signals (blood flow signals) reflected from blood cells. The Doppler signals thus obtained are converted to data of velocity and/or scattering power of blood flow.
More precisely, applying quadrature phase detection to the echo signal at each time of ultrasound scanning with the use of mixers and LPFs (Low Pass Filters) provides an I (In-Phase) signal and a Q (Quadrature-Phase) signal, both of which are extracted as Doppler signals.
The extracted Doppler signals contain by mixture a reflected wave signal from objects in motion (moving elements), such as blood cells, and a second reflected wave signal (called Clatter Signals) from almost non-moving fixed objects, such as the blood vessel wall and organ parenchyma. Of these wave signals, the reflected wave signal from the objects in motion contains a Doppler shift. In contrast, the reflected signal from the fixed objects hardly contains a Doppler shift, and is so high in its intensity that the signal is a dominant in the detected signal.
Therefore, clatter components representing the reflected wave from the fixed objects are eliminated through an MTI (Moving Target Indicator) filter by taking advantage of a difference in the quantity of Doppler shift, a blood flow Doppler signal can be efficiently extracted. Then, through analysis of the frequency of this blood flow Doppler signal, i.e., N-pieces of Doppler data composed of xi (I signal) and yi (Q signal) at each depth (where i=1, 2, . . . , N), a mean derived from the spectra (i.e., a Doppler frequency), a dispersion of the spectra, or a reflection intensity (power) from blood cells can be calculated.
For this frequency analysis, an autocorrelation function is normally used. A frequency analysis technique that uses the autocorrelation function will now be exemplified. As above described, the blood flow Doppler signal obtained by eliminating its clatter components at the MTI filter is expressed by a complex number zi composed of Doppler data xi and yi, each is N-pieces in number, and expressed by the equation of:
                                                                        z                i                            =                                                x                  i                                +                                  jy                  i                                                                                                        =                                                a                  i                                ·                                  exp                  [                                      j                    ⁢                                          {                                                                                                    2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              d                                                        ⁢                                                                                          T                                rn                                                            ⁡                                                              (                                                                  i                                  -                                  1                                                                )                                                                                                              +                                                      ϕ                            ]                                                                          ,                                                                                                                                                                    (                              i                =                                  1                  ,                                                                          ⁢                  2                  ⁢                                      ,                                                                                  .                                                                                  .                                                                                  .                                                                                                              ⁢                                                                        ,                                                                                                          ⁢                          N                                                )                                                                                                                                                    (        1        )            where ai is an amplitude, fd is a Doppler frequency, Trn is intervals of transmission of ultrasound pulses along an arbitrary raster direction, and φ is an initial phase respectively. For the sake of explanatory convenience, it is assumed that the Doppler frequency fd is constant over the N-pieces Doppler data, still maintaining the generality of the equation.
According to the above equation (1), the phase rotation of the complex number zi per unit time provides a Doppler frequency fd. Where a mean complex autocorrelation function for the N-piece Doppler data is:Z=X+jY=A·exp[jη],  (2)the following equation can be obtained:
                                                        Z              =                                                                    (                                          N                      -                      1                                        )                                                        -                    1                                                  ⁢                                                      ∑                                          i                      =                      1                                                              N                      -                      1                                                        ⁢                                                                          ⁢                                                            Z                      i                                        *                                          Z                                              i                        +                        1                                                                                                                                                                    =                                                                    (                                          N                      -                      1                                        )                                                        -                    1                                                  ⁢                Σ                ⁢                                                                  ⁢                                                      a                    i                                    ·                                      a                                          i                      +                      1                                                                      ⁢                                                      exp                    ⁡                                          [                                              j                        ⁢                                                  {                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              d                                                        ⁢                                                          T                              rm                                                                                }                                                                    ]                                                        .                                                                                        (        3        )            The Doppler frequency fd is therefore expressed as:fd=(2π)Trm−1tan−1(Y/X),  (4)X=Σ(xixi+1+yiyi+1)Y=Σ(xixi+1−yiyi+1)
By employing this Doppler frequency fd, the equation of:Vd=fd·c/(2fM·cos θ)  (5)can be obtained, so that a Doppler velocity Vd is converted using this equation (5). In this equation (5), c, fM and θ indicate a sound velocity, the frequency of a reference signal at the mixers, and an angle formed between an ultrasound beam and each blood flow (hereinafter referred to as a “Doppler angle”), respectively.
In the case of a CFM mode, due to difficulty in obtaining Doppler angles at each position in the space of an image, which vary position by position therein, the correction of Doppler angles is omitted from the computation on the foregoing equation (5). In other words, in the CFM mode, the Doppler velocity Vd can be calculated based on the equation of:Vd=fd·c/(2fM)  (6),and is subjected to display in colors. Consequently, where the Doppler angle is larger, a calculated value becomes smaller than its original correct velocity, with the result that the Doppler velocity Vd is subject to display based on color intensities representing slower velocity states (this is called “angle dependency”).
Blood flow velocities obtained as described above are two-dimensionally displayed on a monitor, normally, together with a B-mode topographic image displayed as a background.
In recent years, three-dimensional image display in ultrasonic diagnosis apparatuses has been extensively researched and developed, and it has been possible to three-dimensionally display a power image of blood flow. For such a display, for acquiring three-dimensional data, a hand scanning technique by which an electronic scanning probe with one-dimensionally arrayed transducers is used, for example.
To operate this hand scanning, while being electronically scanned in the direction along the transducer array, an operator moves his or her hand holding the probe so that the probe is moved to orthogonal directions to the transducer array direction.
However, the display on the current CFM mode has encountered the following problems.
First, in recent years, as various types of diagnostic methods have been advanced, there are demands for a display technique that allows a user to identify a blood vessel as an artery, portal vein, or vein in a steadier and easier manner. In particular, to identify a blood vessel as described above by using an ultrasound wave, it is considered effective to observe pulsation appearing in blood flow.
As one conventional examination method for examining pulsatility of this kind of blood vessel, one display method called “pulsatility index (PI)” has been known. The PI is an index that quantifies the extent of change in a blood flow velocity per heartbeat. Since peripheral circulatory resistance in blood vessels is reflected in the PI, it is deemed effective for early detection of dysgenesis of fetuses in the obstetrical department and for differential diagnosis of tumor in the abdominal part (refer to, for example, a Japanese Patent Laid-open (KOKAI) Publication No. H05-317311).
Other conventional examination methods are provided to examine the pulsatility of blood vessels, for example. One method is to display an acceleration of blood flow calculated from two frames of blood flow velocity data which are adjacent in time and stored in a frame memory (Japanese Patent Publication No.2768959). Using this method, information about the pulsation of acceleration of a blood flow can be added on two-dimensional color flow map information or three-dimensional display information based on the CFM mode. Another method is proposed by an ultrasound imaging method and apparatus that is able to display an image of intensity of the pulsation appearing in the moving velocity of an echo source (Japanese Patent Application Laid-open (KOKAI) Publication No.2000-152935). This method comprises the steps of detecting a moving velocity of an echo source based on Doppler shifts of received echoes, detecting intensities of the pulsation appearing in the moving velocity calculated on moving velocities at the current and past temporal phases, and producing an image indicative of the detected intensities of the pulsation. Those methods have not, however, reached a level of practical use yet.
Besides, the present CFM mode has a difficulty to clearly display the pulsatility of blood flow in displaying its power. It is considered that displaying the velocities of blood flow still provides distinguishable observation with respect to the pulsatility. In other words, temporal changes in the colors indicative of velocities shows that there is pulsatility in a blood flow to be observed, while no temporal changes in such colors shows that, there is no pulsatility in the blood flow. However, there are many cases that make it difficult a clear discrimination of blood flows even if carefully watched, thereby still lacking practically. Whichever of the display techniques are chosen, a more convenient display technique is required to provide the pulsatility of blood flow.
Especially in the case of peripheral blood vessels, their blood flow velocities are relatively lower, amounts of changes in the velocities showing the pulsatility are also small, even in arteries. It is therefore considerably difficult to distinguish an artery from a vein or vise versa on a displayed image. In addition, displaying the velocity has the problem of the angle dependency if the Doppler angle is larger, as described before, resulting in that detected velocities are smaller than their original correct velocities. It is therefore very difficult to detect the pulsatility, like the situation in peripheral blood vessels.
On the other hand, in the foregoing three-dimensional display, a further advanced display rather than the simple display of blood vessels is demanded. Such advanced display techniques include a display technique that has the capability of classifying the types of blood vessels, such as artery, portal vein, or vein. For this purpose, it is also considered advantageous that such display involves pulsatile flows of blood, which requires an ultrasonic diagnosis apparatus that is able to three-dimensionally display the pulsatility.